First rule of vowel harmony

The 8 Turkish vowels are divided symmetrically into the
following two groups:

Thick vowels: a, o, u, ı (dotless vowels)

Thin vowels: e, ö, ü, i (dotted vowels – think of “e” as
an “ä” as in German)

Rule:

Normally, a Turkish word contains either thick or thin vowels, but
never both.

All rules applying to one set of (thick or thin) vowels apply
symmetrically also to the other set.

Examples:

Sarılacaklarını (sar-ıl-acak-lar-ı-nı) (that they will be rolled in)

Serileceklerini (ser-il-ecek-ler-i-ni) (that they will laid down)

Exceptions:

In composite words (words consisting of two or more words combined
together) the rules of harmony apply to the sub-words separately. Example:
kazandibi (kazan+dibi,
meaning: the bottom of the pan) (a Turkish desert)

Words of foreign origin, eg: fiyat (price), televizyon.

The “continuous tense” suffix “-yor” (because “yor” probably used to
be a separate verb, meaning “to be busy with” – also related to the verbs
“yormak” (make tired) and yorulmak (become tired)).

Note that the symmetry simplifies the 2nd rule of
vowel harmony, because the rules which apply to the thick vowels apply
symmetrically also to the thin vowels.